A concept device by designer Yasuhiro Akama would be pretty handy to have while walking in cities. It uses different colors of LED lights to alert users to the quality of the air they're breathing.
The little clip-on gadgets would measure the quality of the air and light up either red for dirty air, white for "normal conditions," or green for clean air.
It's a little disturbing that "normal conditions" aren't considered clean air, but for urbanites, I guess that is indeed normal. For now, though, AIRNow's mashup will have to suffice.